This invention relates to a process for manufacturing solar cells by diffusion, and more particularly to a process of phosphorus diffusion for manufacturing solar cell.
Solar energy is an inexhaustible and renewable energy source for mankind, and also a clean energy, so utilization of solar energy can effectively reduce environmental pollution. The use of photoelectric effect of solar energy is a field that is the subject of much research, and one of the fastest growing and most dynamic research fields in recent years. Solar cells are manufactured mainly based on semiconductor materials, and its working principle is that the photoelectric materials absorb solar energy and produce electrical currents through photoelectric conversion. Silicon solar cells are commonly used today, and are classified into mono-crystalline silicon cells, amorphous silicon solar cells, poly-silicon solar cells and so on. Among them, mono-crystalline silicon solar cells have the highest conversion efficiency and its technology is the most mature.
In previous art, methods for manufacturing mono-crystalline silicon solar cells includes the following steps: (i) peeling off the affected layer; (ii) etching; (iii) diffusion and junction making; (iv) plasma etching of edge; (v) removing phosphorus glass; (vi) apply anti-reflection coating; (vii) screen printing; and (viii) sintering. The diffusion and junction making (usually phosphorus diffusion and junction making) is a key step, and the quality of this step will affect the photoelectric conversion efficiency. In a commercial process, the typical steps of junction making are: first, pass nitrogen through liquid POCl3, transfer the impurity for doping onto the surface of a high-temperature semiconductor by using carrier gas, where the depth of diffusion of impurities is about hundreds of nanometer; then, treat with high temperature so that the impurity atoms pre-deposited on the surface continue to diffuse into deep base and thus form a N+/N layer, where such a structure is conducive to making the electrode in subsequent steps. Such emission zone structure can also be obtained with a one step diffusion method, but it needs to add a corrosion process; a fast heating process can also be used for said junction making, which can greatly simplify the process, but this process is only at an early stage.
However, the above diffusion and junction making process is based on the use of high quality mono-crystalline silicon, which is usually required to have purity of more than 6 N. But the more the purity, the more expensive the material cost, which makes the price of mono-crystalline silicon solar cells high and is hard to put into universal application. At the same time, by using raw materials of metallic silicon with low purity as a starting material, after melting and refining, unidirectional solidification refining, and so on, the purity can reach 4 N˜5 N, but to further purify the material to get more than 6 N high-purity silicon, it would greatly increase the cost, and also reduce the utilization of silicon. Therefore, the development of a phosphorus diffusion method to directly use 4 N˜5 N low-purity silicon material to make solar cells with better conversion efficiency will have great significance.